This invention relates to laminar pellets of crystalline ethylene terephthalate polymer film and to improved processes for the solid-phase polymerization and drying of poly(ethylene terephthalate), and hereinafter PET, and more particularly relates to the solid-phase polymerization of PET waste materials.
Considerable quantities of waste are accumulated during the manufacture of PET film. For instance, startup waste, bead and slitter trim, and reject film may comprise up to 50% of a biaxially oriented, PET film production run. The industry has proposed a variety of methods for reclaiming or recycling these wastes to improve process economics.
Linear polyesters stored under atmospheric conditions for several days absorb up to 0.4% or more of their weight of water. Upon remelting such polymer for its recovery or reuse, this absorbed water can cause a loss of up to 20% of its initial intrinsic viscosity (i.e., a substantial loss in molecular weight). Accordingly, most commercially acceptable processes for recovery or reuse require that the polymer be dried or be further polymerized, or both, before melting to prevent the viscosity from falling to below acceptable levels for fiber of film formation.
Because the rate of drying and polymerization depends upon the rate of diffusion from the bulk of the polymer of volatile substances, specifically, absorbed water, and glycol and water liberated from the condensation reaction as the polyester is further polymerized, it has been the practice to grind the film or other polyester material to a fine state. Classically, in U.S. Pat. No. 2,503,251, Edwards et al., which teaches the necessity for drying to maintain viscosity upon melting, the polymer is ground to a fine powder. More recently, in U.S. Pat. No. 3,657,388, Schweitzer et al., it is taught to compress the polymer in a "powdery or very fine-grained" state into particles. Processes for converting the polymer into these finely divided states are costly in energy consumption and can cause unwanted side reactions to produce color and the like in polymers such as polyesters. The present invention avoids the foregoing and enables the conversion of low bulk density waste crystalline polyester film into a conveniently handled state, with a minimum sacrifice in rates of drying, of polymerization in the solid state, and of dissolution times in molten polymer and in hot glycol-monomer systems for glycolysis to monomer.
U.S. Pat. No. 3,767,601 to Knox discloses a promising method for reclaiming general-purpose PET film waste, typically having an intrinsic viscosity of about 0.50 to 0.56, by comminuting the waste to flake form and then subjecting the flske to solid-phase polymerization, in the presence of a scavenging gas, to increase the PET intrinsic viscosity. The resulting waste can be reprocessed by melt extrusion to make products requiring the properties associated with high molecular weight PET. For instance, reclaimed PET having an intrinsic viscosity of slightly above 0.70 can be used to make a heat-sealable, heat-shrinkable film.
It has been found, howewer, that thin-gauge PET flake has such a low bulk density, typically in the range of 2.5 to 5 pounds per cubic foot (40 to 80 kilograms per cubic meter), that an excessively large reactor or dryer is needed to achieve practical production levels. Moreover, the low bulk density may cause material handling problems which limit process throughput. For example, it is difficult to sufficiently agitate a large charge of low-density flake to achieve uniform exposure to a scavenging gas, such as in the rotary kiln of Knox or a low pressure environment, and the low-density flake may clog passageways or become entrained in the scavenging gas stream, thereby clogging venting ports through which the scavenging gas is discharged.
To overcome these problems, one might melt extrude the comminuted wastes into solid granules or pellets which would then be subjected to solid-phase polymerization. But the reaction rate is limited by the relatively slow diffusion of ethylene glycol and water by-products to the waste surface where they envolve. Consequently, the use of larger sized waste sacrifices reaction rate and, as a practical matter, increases the plant investment required to achieve a given production rate.
Thus, there is a need for an improved solid-phase polymerization process which will give polymerization rates attainable with finely divided PET wastes, but which will not present the practical problems associated therewith.